Actuators of the type mentioned by way of introduction are known from, for instance, the applicant's own U.S. Pat. No. 7,121,237. Said document discloses an actuator for the driving of an engine valve, wherein the actuator piston rod of the actuator piston has a hydraulic braking device in the area of the free end thereof, which hydraulic braking device interacts with a mechanical stop in the actuator housing of the actuator. The object of this hydraulic braking device is to reduce the movement speed of the engine valve just before the valve head of the engine valve contacts the valve seat in the cylinder of the combustion engine, and thereby obtain a controlled closing motion, in order to spare the included details and reduce wear and dissonance. Upon closure of the engine valve, the actuator piston rod should contact a mechanical stop in the actuator housing together with the engine valve contacting the seat of the same, in order to obtain correct braking of the engine valve and actuator piston in connection with closure of the engine valve.
It is utmost important that the mutual distance between the hydraulic braking device of the actuator piston rod and the valve head of the engine valve is as large as the mutual distance between the mechanical stop and the seat of the engine valve. The problem of known actuators is that the actuator piston rod does not reach its mechanical stop in the actuator housing, whereupon the retarding effect varies or does not appear at all.
Because the engine valve is exposed to high temperatures during operation, the valve stem of the same will undergo dimensional changes, which directly affects the condition above. In addition, the manufacturing costs will be high if the dimensional tolerances in the production of the included details are narrow, alternatively less narrow dimensional tolerances are used, which entails a need of using shims or the like to adjust the mutual positions of the details. Furthermore, the included details are worn during operation, which further affects the mutual positions of the details.
It should be mentioned that the actuator piston rod, besides the hydraulic braking device, also has other functions where the position of the same in relation to the actuator housing is used for different purposes, for establishing the size of the first part of the cylinder volume, for regulating the fluid communication with the first part of the cylinder volume, etc.